Gas turbines, as are used as stationary machines for example in combined cycle power plants, compress air which is inducted in a compressor section and direct the compressed air to a combustion chamber where it is used for combusting a fuel. The hot combustion gases are expanded in a subsequent turbine, performing work, and then discharged to the outside or directed through a heat recovery steam generator for producing steam. The blades in the turbine section, which are divided into rings of (stationary) stator blades and (rotating) rotor blades, which are arranged in series in an alternating manner, are exposed to high thermal and mechanical stresses as a result of the extremely hot gases which flow in the hot gas passage of the turbine. In order to achieve a turbine inlet temperature which is as high as possible, and therefore high efficiency, special materials and cooling techniques are used for the blades of the turbine.
The first row of stator blades directly after the outlet of the combustion chamber is particularly stressed because the temperatures of the hot combustion gases are highest there. In FIG. 1, an exemplary stator blade from the first row of a gas turbine which is in the market is shown in a perspective side view. The stator blade 10 comprises a blade airfoil 12 which extends in the longitudinal direction of the blade, or in the radial direction of the gas turbine, and at the ends merges into an inner platform 13 and an outer platform 11. The inner platforms 13 of all the stator blades 10 of a ring together form an annular inner wall which delimits the hot gas passage 17 on the inside. All the outer platforms 11 correspondingly form an annular outer wall which delimits the hot gas passage 17 on the outside. In preferred regions of the platforms 11, 13 and of the blade airfoil 12, holes or hole rows are arranged, through which cooling air flows into the hot gas passage 17 and forms a protective cooling air film there.
The stator blade 10, or its blade airfoil 12, however, is also cooled from the inside. For this, the blade airfoil 12 has a hollow inner space (18 in FIGS. 2 and 3) through which cooling air is directed and in a defined manner brought into contact with the inner sides of the blade airfoil walls. To accommodate this, a multiplicity of hollow inserts 14, 15 and 16 are provided with hole rows, the inserts can be inserted into the inner space 18 of the blade airfoil 12 in the longitudinal direction and from the cooling air, which is fed in its interior, form individual cooling air jets which are directed onto the inner sides of the blade walls for impingement cooling. In order to divide the inner space 18 into individual compartments, sealing grooves 19, into which the corresponding insert 14 with a matching rib 20 (FIG. 3) can be inserted, are formed on the inner side of the blade wall and extend in the longitudinal direction of the blade (FIG. 2).
The sealing groove 19, which is shown in FIG. 2, is located directly in the region of the leading edge of the blade and as a result is thermally particularly stressed. Therefore, after an extended operating time this groove frequently loses its original shape and integrity so that the intended sealing function in interaction with the rib 20 of the insert 14 is no longer ensured and cooling of the blade no longer functions in the customary manner. In order to still be able to reuse the blade, it would be desirable if the blade could be reconditioned with limited expenditure so that it regains its original functionality again to the full extent.